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Identifying available connections
The first step in connecting your computer to a TV or projector is finding an identical port on both machines. Once the matching port has been identified (one that is the same on both your input device and output device), you need the appropriate cable to connect them. This section contains a picture of the back of an Epson projector and its available connection ports; as well as descriptions of each.

HDMI
The HDMI connection is very popular among display electronics. In fact, nearly all modern televisions and projectors feature HDMI ports. Most laptop computers support HDMI and it is becoming more prevalent on both desktop as well; even without a high-end video card. HDMI is quickly becoming the standard for all electronic equipment for its high quality signal and ability to carry both audio and video signals. The image to the right is that of an HDMI cable.

VGA
The VGA connection is the most common among both desktop and laptop computers, is found on most projectors, and some TVs. The VGA cable has a 15-pin connector on each end that plugs into a VGA port on each device. Due to the fact that most televisions do not support VGA, we recommend using HDMI for their wider range of compatibility.

Tip:VGA cabling is universal for devices that support it. For example, the if your desktop monitor that uses a VGA cable it, that same cable can be used by a laptop to connect it to a projector.
Note: If you are using an Apple desktop or laptop you need a VGA adapter to connect a VGA cable to the computer.

DVI
The DVI connection is newer than VGA and it offers a sharper image. Although the DVI port is not shown on the Epson model above, it is still somewhat common for projectors, not so much for televisions. It is mostly found among desktop computers for monitors, but some laptops have DVI connections as well (Apple laptops are more commonly known to support DVI than any other brand of laptop). Since finding DVI on a TV or projector is more difficult, we again recommend using HDMI cables.
Tip: There are special cables that convert from DVI to VGA or DVI to HDMI and vice versa.

Composite Video
The composite video connection is quite common on a TV or projector, but it is nearly nonexistent on modern computers. This connection is the yellow female cable on what is normally a three bundle of red, white, and yellow. The only time you should see this setup is on older video cards for desktop computers.
S-Video
The S-Video connection is also commonly found on TVs and projectors, but solemnly on a desktop or laptop computer. This connection is a small step up from composite video, but is nearing obsolescence.

Connecting computer and projector or TV
After you've identified what connections are available on both your computer and TV or projector, you're ready to connect the cables. If the same connections are not available for both the computer and TV or projector, you'll need to purchase a video converter cable that converts one signal into a compatible signal.

Desktop Computers
For a desktop, you simply need to plug the cable into the computer and output device. If you don't see an image, you may need to change the display using the following steps.

Press the Windows Key.

Type Adjust screen resolution and press Enter.

Find Display and click the down arrow on the right-hand side of the box.

Laptop Computers
If you're connecting a laptop computer to a TV or a projector you'll often need to "send" the video signal to the display device. The key sequence to do this varies depending on the laptop; but usually it's either: Fn + F3, F4, F5, F8, or F9. For example, pressing and holding Fn + F3 at the same time on my laptop sends the video signal to my connected TV instead of the laptop's screen. The corresponding key used with Fn may be labeled as CRT/LCD or have a picture of a monitor on or close to the key. Additional help and information with switching the laptop display can also be found on the link below.

Switching TV inputs
Finally, if you're connecting a computer to a TV make sure it has been switched to the correct input. For example, if you connected an HDMI cable to your computer and the "HDMI 2" port on your TV, you'll need to switch to the "HDMI 2" input. This action can be accomplished by pressing the input button on your TV remote until the correct image is displayed.

1. Is your display automatically transfer to the projetor right away when you plug the VGA/HDMI cable to the computer? If not, and Fn+F8 doesn't do anything, try to press the Windows Key+P (the windows key looks like this ). This will allow you to switch between displays. (see image below).

2. If you choose one of the display mode above and it still won't clone/extend the image on your computer to the projector, try to use a different cable. If still no go, try to re-install the display driver and see if that fixes it. If same issue, there might be a problem with your VGA/HDMI/Display port and needs to be serviced.

just connect the vga cable to vga port of laptop and to the projector and in projector choose the source of display vga or pc mode and in laptop press f5 button or right click on desktop and choose the display mode lcd +monitor it will show you the display on projector.Have a nice day and do not forget to rate please...!

Try the following:
Power up the projector first. With the laptop hooked up but not powered on once projector is warming up begin booting laptop. If there is still no display once its projector is fully warmed up and laptop booted you can try using the (fn) key with the laptop monitor switching function key usually F4 which switches the display monitor to show on external, external and laptop or just laptop. If this all fails try changing resolution of laptop to something that might be more projector friendly.
Let me know how you go.

There will be a function key that allows you to swap what monitor the display comes up on. On my HP its the blue (fn) function key with f4. This should swap display between laptop monitor and external or show up on both.
Other things to check are that the projector is powered on and plugged in before the laptop powers up.
Also check resolution of laptop display it may not be compatible with the projector.

I have encountered this same problem once in the office when i was trying to do the same thing.

The solution:*Assuming you have windows xp*

1) make sure the projector and the laptop are connected (FN+F8), do a right click anywhere in your desktop then go to settings by default Monitor 1 would be your Lattitude D620 and Monitor 2 should be your projector, to make sure of that click on the 'identify' it should show a 1 in your laptop and a 2 on the projector2) now select monitor 2 and select 'use this device as my primary monitor' check mark, by doing this your videos should play in your monitor but they will be a blank/black screen on the laptop (basically we are reverting the order).

first, I would check screen cable. try replacing it with a new one, you will be surprised to know how many display problems are are caused by a defected screen cable.in case this is not the problem, it can get more complicated, but first, just in case, reinstall drivers. its a long shot, but if you get lucky it will save you alot of pain. in case it didn't help, It's probably a bad vertical output chip. if you have some familiarity with monitors and decent soldering skills you could probably fix it yourself, but I would advise looking into taking it to a shop, or at least find an experienced hobbiest if you're not yourself. It's a pretty straightforward fix for someone who knows what they're doing.